According to Arris, “Wi-Fi printer(s) may lose the Wi-Fi connection after going into sleep mode with some period of inactivity. Although the printer was working normally after installation, it cannot successfully reconnect to a secure W-Fi network after coming out of sleep mode and is activated for use. The cause for the loss of connectivity has been isolated to an incompatibility issue with some Wi-Fi printers and how they maintain a secure Wi-Fi network connection to the Wi-Fi Gateway.
Power cycling or resetting the Wi-Fi printer will usually solve the problem and needs to be performed each time the Wi-Fi printer awakes from sleep mode. If the power cycle doesn't resolve the issue, use the following workaround. The workaround consist of creating open Guest Wi-Fi Network with MAC address filter in the SBG6900-AC to allow connection only for the Wi-Fi printer.
https://arris.secure.force.com/consumers/articles/General_FAQs/SBG6900-AC-v1-5-x-Printer-Wi-Fi-Connectivity-Workaround/